1. Field of the Invention
The present invention relates to the preservation of eye tissue in a defined nutritive. aqueous medical solution, and more particularly, relates to the preservation and enhancement of human corneal tissue, specified as the time between removal from the donor and transplantation.
2. Description of the Prior Art
Keratoplasty, or transplantation of the cornea, has been effective in providing visual rehabilitation to many who suffer from corneal disorders. This procedure has gained widespread acceptance but has been severely hampered by the universally inconsistent availability of donor tissue. This problem made the development of a storage solution imperative. The development of MK.TM.-preservation medium, and subsequent chondroitin sulfate-containing media, has positively impacted the availability of quality donor tissue. Much research in this area has been undertaken with a view towards prolonging donor storage time and yet maintaining a viable endothelium, which is crucial to successful transplantation. Storage of the cornea for up to 14 days at 4.degree. C. has been reported, although the current technology does not permit adequate tissue preservation beyond a few days. Storage longer than 96 hours is attended by epithelial decomposition and loss of corneal clarity, as demonstrated by increased swelling of the corneal stroma. This stromal edema is attributed to the decreased maintenance of the barrier pump function of the corneal endothelium, a specific cell layer lining the corneal stroma.
The functional status of the endothelium and sustained corneal deturgescence after corneal preservation are of great clinical importance, and contribute primarily to the success of the surgical outcome. The ability of the cornea to maintain a relatively dehydrated state is essential to the maintenance of corneal transparency. Corneal deturgescence is an energy-dependent phenomenon performed primarily by the endothelial cells. In order for the cornea to remain viable, various enzymatic reactions must occur to carry out energy-dependent functions, maintained by high levels of ATP.
The lower temperature of the 4.degree. C. corneal storage method reduces the metabolic rate of the cornea, but the storage medium must still be able to support the basal requirements of the cornea. Thus, corneal storage media are a complex mixture of balanced salts, amino acids, energy sources, antioxidants, buffering agents, cell membrane stabilizers, glycosaminoglycans, deturgescents and antibiotics. Temperature reduction changes the membrane lipids, proteins and water structures, each of which could alter the active transport mechanism by hindering the ease of passive diffusion, carrier-substrate interaction and energy-coupling relationships. Thus disturbances of membrane function, as well as morphological and biochemical alterations, assume a greater consequence as the direct result of the lower metabolic rate. Therefore, a critical evaluation of physiologic parameters such as ionic and amino acid composition, bicarbonate equilibrium, available energy sources, dissolved oxygen levels, osmolality and pH should be observed with respect to each preservation medium. Parameters for extended 4.degree. C. storage should be defined as to the reversibility of cell damage incurred during storage.
Adult corneal endothelium have a limited regenerative capacity and mitotic figures have been rarely observed in vivo; human corneal endothelium in vivo normally responds to trauma by sliding into the wounded area by cell migration. However, in vivo endothelial cell mitosis has been demonstrated in rabbits, cats and primates. In tissue culture, mitosis has been observed in rabbits and human corneal endothelium. Autoradiographic thymidine uptake studies after cryowounding or mechanical wounding of corneas in vitro has demonstrated existence of mitotic figures in the endothelial monolayer. Surgical trauma and disease can accelerate the loss of endothelial cells and further compromise the cornea. Thus, the long term preservation and enhancement of the corneal endothelium is a very important aspect of eye bank storage of eye tissue.
An overview of the issues surrounding the storage and handling of corneal tissue is found in Corneal Surgery, Chapters 1-4, pages 1-128 edited by Federick S. Brightbill, M.D., published by C. V. Mosby Company, St. Louis, Mo. 1986. A variety of storage media and techniques have been proposed, and current research continues to be directed towards maintaining and actually enhancing the quality of donor tissues, and increasing the duration of storage corneal tissues, as defined as the time between excision from a donor and transplantation.
Accordingly, the present invention is directed toward materials and methods for enhancing ocular tissues, especially corneal tissues, during storage prior to transplantation. One aspect of the invention provides for the enhancement of corneal tissue viability by maintaining normal physiologic metabolism and corneal deturgescence during low temperature storage. Another aspect of the invention provides for increasing the length of time that eye tissues, especially corneal tissues, can maintain the attributes of fresh tissue.